1. Field of the Invention
The present invention pertains, in general, to a propeller shaft of rear-wheel drive vehicles and a method of manufacturing the same and, in particular, to a propeller shaft, in which composite material prepregs are stacked to an inner surface of a metal tube to increase axial stiffness and fundamental natural vibration frequency of the propeller shaft and prevent its physical properties from being degraded due to external impact, and a method of manufacturing the same.
2. Description of the Prior Art
Used to transmit a rotating force from engines of rear wheel drive vehicles to rear axles, a propeller shaft conventionally consists of steel or aluminum materials. However, recently, a composite material has been used instead of the conventional steel or aluminum material as a material of the propeller shaft so as to reduce weight and improve NVH (noise, vibration, and harshness) properties of the propeller shaft, and the propeller shaft consisting of the composite material has begun to be practically installed in vehicles.
The fiber reinforced composite material such as carbon fiber polymeric matrix composites is better than the conventional steel or aluminum material in terms of specific stiffness and specific strength. Accordingly, the propeller shaft consisting of the composite material has a higher axial specific stiffness when the fiber direction is close to the axial direction than the conventional steel or aluminum materials, so its fundamental natural bending vibration frequency is higher than those of conventional materials.
However, the propeller shaft consisting of only the high performance composite material is less competitive in terms of production cost in comparison with the propeller shaft consisting of the conventional steel or aluminum material because large amount of composite material should be used in order for the propeller shaft made from the high performance composite material to have torque transmissibility sufficient to transmit the power from the engines of the vehicles to the rear wheel axles with its fundamental bending natural frequency higher than the engine rotational speed.
Much effort has been made to avoid the above disadvantage, that is to say, undesirable increase of production cost of the propeller shaft. For example, reference may be made to the patents assigned to the applicant of the present invention, Korean Pat. No. 103245, which discloses a process of manufacturing a propeller shaft consisting of a light metal material with excellent torque transmissibility (e.g.: an aluminum alloy) and a composite material with excellent specific stiffness (e.g.: unidirectional carbon fiber epoxy composite material).
The process comprises the steps of sequentially rolling a vacuum bag and a composite material prepregs around an outer surface of a mandrel, inserting the resulting mandrel into a metal tube to stack the composite material prepregs to an inner surface of the metal tube, applying high pressure air into the vacuum bag after the mandrel is removed to allow the composite material prepregs to come into close contact with the inner surface of the metal tube, and curing the composite material prepregs for a predetermined time to co-cure bond the composite material prepregs to the inner surface of the metal tube. However, this process may induce high thermal residual stresses at the interface between the composite material and the metal tube because the composite material and the metal tube have different thermal expansion coefficient values, thus causing the composite material prepregs to readily peel off from the inner surface of the metal tube upon receiving even weak external impact and allowing moisture absorption between the composite material and the metal tube, which degrades physical properties of the propeller shaft.
To avoid the above disadvantages, a high-priced waterproofing epoxy and urethane paint may be coated on the hybrid propeller shaft consisting of the metal and composite material to reduce moisture absorption between the composite material sheet and the metal tube and improve impact resistance of the propeller shaft. However, the propeller shaft coated with the above paints has a disadvantage of high production cost and increase of weight.
Meanwhile, the propeller shaft may be manufactured by stacking the composite material prepregs to the inner surface of the metal tube to form a power transmission shaft, and fitting metal yokes around both ends of the transmission shaft. In this regard, Korean Patent No. 197355 registered to the applicant of the present invention discloses a process of fitting a transmission shaft into metal yokes.
In Korean Patent No. 197355, the transmission shaft consisting of a metal tube and a composite material prepregs is fitted into the metal yokes with the use of an adhesive to manufacture a propeller shaft, but this propeller shaft is disadvantageous in that its torque transmissibility and natural vibration frequency are poor and a center of the transmission shaft is easily deviated from a desirable position. Also the propeller shaft consisting of a carbon fiber and aluminum tube with the yokes welded to the aluminum tube is poor in terms of reliability because the carbon fiber and epoxy resin are sensitive to heat when the metal yokes are welded to the transmission shaft.
Therefore, in Korean Patent No. 197355, the transmission shaft is fitted into the metal yokes after serrations are precisely formed on the inner surfaces of both ends of the transmission shaft and outer surfaces of the metal yokes. Thereby, the propeller shaft including the metal yokes forms an integrated body unlike a conventional propeller shaft composed of two pieces each having a length of 1.5 m, so it is not necessary to use a conventional yoke assembling two pieces with each other and positioned at an interface of the two pieces, bearings for supporting the shafts, and vibration-proof rubbers.
However, the propeller shaft is disadvantageous in that there is a need for sophisticated precision processing so as to desirably engage the serrations of the transmission shaft with those of the metal yokes, thereby undesirably increasing production cost and time of the propeller shaft.